mount: mark an existing "mounting" unit from /proc/self/mountinfo as …

…"just_mounted"

When starting a mount unit, systemd invokes mount command and moves the
unit's internal state to "mounting".  Then it watches for updates of
/proc/self/mountinfo.  When the expected mount entry newly appears in
mountinfo, the unit internal state is changed to "mounting-done".
Finally, when systemd finds the mount command has finished, it checks
whether the unit internal state is "mounting-done" and changes the state
to "mounted".
If the state was not "mounting-done" in the last step though mount command
was successfully finished, the unit is marked as "failed" with following
log messages:
  Mount process finished, but there is no mount.
  Failed with result 'protocol'.

If daemon-reload is done in parallel with starting mount unit, it is
possible that things happen in following order and result in above failure.
  1. the mount unit state changes to "mounting"
  2. daemon-reload saves the unit state
  3. kernel completes the mount and /proc/self/mountinfo is updated
  4. daemon-reload restores the saved unit state, that is "mounting"
  5. systemd notices the mount command has finished but the unit state
     is still "mounting" though it should be "mounting-done"

mount_setup_existing_unit() should take into account that MOUNT_MOUNTING
is transitional state and set MOUNT_PROC_JUST_MOUNTED flag if the unit
comes from /proc/self/mountinfo so that mount_process_proc_self_mountinfo()
later can make state transition from "mounting" to "mounting-done".

Fixes: systemd#10872
(cherry picked from commit 1d086a6)

src/core/mount.c

@@ -1568,7 +1568,7 @@ static int mount_setup_existing_unit(
         if (r > 0)
                 flags |= MOUNT_PROC_JUST_CHANGED;
 
-        if (!MOUNT(u)->from_proc_self_mountinfo || FLAGS_SET(MOUNT(u)->proc_flags, MOUNT_PROC_JUST_MOUNTED))
+        if (!MOUNT(u)->from_proc_self_mountinfo || FLAGS_SET(MOUNT(u)->proc_flags, MOUNT_PROC_JUST_MOUNTED) || MOUNT(u)->state == MOUNT_MOUNTING)
                 flags |= MOUNT_PROC_JUST_MOUNTED;
 
         MOUNT(u)->from_proc_self_mountinfo = true;